Ok i was looking on an ezyer way to do it and i found this
plz try it see if it works i couldnt figyouut

NOTE i do not take any cedit

Tool's Needed
1.HxD
2.Hexadecimal Converter- [ Register or Signin to view external links. ]
3.Transfer Cable
4.Siggy.
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1. Go to multiplayer on MW2
2. Go to barracks and WRITE DOWN your total amount of XP.
3.Plug in your HDD to your transfer cable
4.Put your account on your desktop(AND MAKE BACKUP)
5.**IMPORTANT** Decrypt your account in siggy.
6.Then go to the hexadecimal converter convert your amount of XP to a heaxadecimal code.
7.Put that code in a notepad.
8.for however man numbers long your XP was (ex.Mine was ten #s long) go to the Hexadecimal converter and type in 9 the same length in #s as your XP
9. Convert that # to hex.
10. Open your account in HxD
11. Search for your XP code (ctrl f) and Make sure you put the type as a hexidecimal
12.MAKE SURE YOU SEE SOMETHING RELATED TO MODERN WARFARE2 BY IT!
13.Replace your xp code with the code that gives you 999etc XP.
14.Save it.
15.Encrypt it (using siggy)
16.Open it in Hash Block Calculator.
17.Make sure everything is true (if one is wrong, you messed up)
18.Rehash & Resign
19.Put it back in your HDD.
20.Go play one game of whatever you want on mw2
21.MAKE SURE YOU GET AT LEAST ONE KILL!
22.Once you have your kill leave
23.Then have fun being a Tenth Prestige Level 70!